Smoke from wildfires blankets a section of the Wasatch Mountains on Thursday, Aug. 9, 2018.

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INDIANOLA, Sanpete County — Evacuated residents were allowed to return home Saturday night after firefighters fully contained the Hill Top Fire, while officials continue to feel the effects of the state's fire season.

The fire, which started Aug. 6 from an unknown cause, displaced 350 residents and destroyed two structures at its peak. The evacuation order was lifted at 10 p.m. Saturday after officials fully contained the fire that consumed over 1,800 acres.

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Trees burn in the Coal Hollow fire on Thursday, Aug. 9, 2018.

Dollar Ridge investigation

After burning through more than 65,000 acres, 438 structures and costing more than $19 million to fight, fire investigators still do not know what caused the Dollar Ridge Fire early last month.

"We get fires every year that we say are undetermined because there's not enough evidence, there's no witnesses and (they're burning) in a very remote area like this one is," said Jason Curry, spokesperson Utah Division of Forestry, Fire and State Lands.

With no storms in the area at the time it ignited, officials believe the fire was caused by humans. Curry said investigators will continue investigating the cause and will look into all possibilities.

"It's frustrating to have the investigation have zero results, especially one that takes homes and impacts communities the way the Dollar Ridge Fire did," Curry said.

A wildfire on Lake Mountain in the Saratoga Springs area, known as the Iron Hill Fire, ignited Saturday and was 60 percent contained by about noon Sunday. Flames at one poin threatened nearby communications towers.

Firefighters worked through the night into Sunday in an effort to secure a perimeter and protect the communications towers. A crew from Nevada was on scene.

The fire was measured at approximately 1,015 acres in size as of about noon Sunday, according to Northern Utah Interagency Fire Center dispatcher Greg Blank.

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Smoke from wildfires blankets a section of the Wasatch Mountains on Thursday, Aug. 9, 2018.

Utah County Sheriff's Sgt. Spencer Cannon said the fire was caused by some target shooters who were using an authorized area for the activity and were the first to report the fire. Cannon doesn't anticipate police will recommend any charges against them.

Road closure, evacuations

The lightning-caused Coal Hollow Fire was just 4 percent contained Sunday morning, and sitting at about 19,360 acres in size with nearly 400 firefighting crew members on the job to combat it.

The fire was worsened by thunderstorms Sunday afternoon when it crossed U.S. Highway 6, shutting down the road for the second time in a matter of days, Utah fire officials reported on Twitter.

The Coal Hollow Fire is burning about 15 miles southeast of Spanish Fork.

Late Sunday, Cannon said some residents and campers were being evacuated in the Diamond Fork Canyon area, as high as Springville Crossing about 14 miles in, affecting a somewhat triangular area encompassed by Diamond Fork Road, Sheep Creek Road and Highway 6.

Cannon said there are "maybe a couple dozen homes" there, as well as cabins and campsites.

"The biggest concern is campers," he said. "That's primarily what you're going to find up Diamond Fork (Canyon)."

Cannon didn't immediately have information about whether there were also evacuations in place Sunday night in areas just south of Highway 6.

Smoke in the air

State officials are forecasting poor air into Monday, advising people to avoid spending too much time outside. Fires from across the state and California will continue to diminish air quality in Salt Lake County into the week.

In both Salt Lake City and Provo, Sunday's air quality was designated as outright unhealthy, a distinction that prompting a government warning that the general population "may begin to experience health effects" from the conditions. The designation was expected to continue Monday in Provo, but downgrade to "unhealthy for sensitive groups" in Salt Lake.

Other areas expected to have air considered unhealthy for sensitive groups on Monday are Brigham City, Logan, and Ogden.

Children, older adults, those with asthma and active people are discouraged from exercising or spending too much time outdoors. Everyone else is also warned against spending too much time in the smoky haze that has blanketed the Wasatch Front.